1. Field of the Invention
This invention pertains to an apparatus and method for improving the performance of vacuum deposition systems. More particularly, the invention relates to an electron beam evaporator mounted onto a moveable stage capable of depositing layers of coatings uniformly onto substrates of various sizes during in-line processing.
2. Discussion of the Art
The coating of substrates within a vacuum has been recognized for some time as being a highly advantageous technique in many applications. For example, vacuum coating may be used for achieving anti-reflection and anti-abrasion properties, non-metallic coatings, zinc or aluminum coated papers and plastics, refined high pressure metals, and various other applications where high purity or unusual alloy compositions are desirable.
One type of coating system includes a vacuum tight enclosure or chamber connected to a vacuum pump. Inside the chamber is a vapor source generally consisting of a crucible or boat. The crucible is water-cooled and contains an evaporant, such as molten metal, with which the substrate is to be coated. The substrate is also housed by the vacuum chamber and is usually supported by holders. The evaporant is heated within the crucible producing a vapor cloud which acts as a transport media to deposit the evaporant on the substrate.
Conventional coating systems use either a resistance heated element or an electron beam to heat the evaporant in the crucible. Electron beam evaporation is preferred over resistive evaporation due to its ability to operate at lower ambient pressures during deposition, its superior deposition rate control, and its multiple source capability. In a coating system utilizing electron beam evaporation, an electron gun is disposed within the vacuum chamber which generates an electron beam. The electron beam is directed toward the evaporant thereby heating the material and producing a vapor transport cloud.
Although electron beam evaporation has several advantages, there is at least one significant drawback. During electron beam evaporation, the evaporant can be approximated as a point source at the substrate surface. What this means is that the amount of material deposited on a substrate per unit of time (evaporation rate) is inversely proportional to the square of the distance from the source to the substrate. Thus, in order to obtain a uniform coating across the entire substrate the substrate assemblies must be hemispherical in shape. A substrate assembly having a hemispherical shape allows each portion of the substrate to be positioned an equal distance away from the source thereby achieving a uniform evaporation rate and uniform coating across the entire substrate.
However, hemispherical substrate assemblies do not lend themselves to "in-line" processing which has significant advantages over "batch-type" systems. When utilizing a batch-type system, the main vacuum chamber must be evacuated and vented to atmosphere after processing each batch of substrates. This has caused serious production delays due to the significant time it takes to evacuate the chamber vacuum after each use. In contrast, during in-line processing the substrates are placed on carriers and continuously conveyed through the system in an expeditious manner. A minimal amount of labor is needed to operate such a system. As noted above, conventional hemispherical substrate assemblies used in conjunction with electron beam evaporation cannot obtain the advantages afforded by in-line processing. In-line processing is not possible in these systems because the substrate must remain in a neutral position in order to achieve uniform coating.
Unlike traditional electron beam evaporation, RF and DC sputter deposition are capable of being used in conjunction with in-line processing. In use, conventional sputter deposition techniques provide two plates having an inert gas, such as argon, disposed between them. On one plate is mounted a substrate and on the other is a target material containing the coating material. A high radio frequency (RF) is impressed across the two plates causing the inert gas atoms to become ionized. The ionized gas atoms strike the target plate containing the coating material knocking off molecules which are then deposited on the substrate. Since sputtering is capable of being used in conjunction with in-line processing, it is presently the technique of choice for coating large flat panel displays.
However, RF and DC sputtering do not have some of the significant advantages offered by electron beam evaporation. In particular, electron beam evaporation has excellent deposition rate control, multiple source ability, and is capable of operating at lower ambient pressures. In addition, sputtering has a higher cost of equipment and operation as well as a slower rate of deposition when compared to electron beam evaporation.
Thus, a need exists to provide a vacuum coating device using electron beam evaporation that is capable of applying a uniform coating on a substrate while at the same time obtaining the significant benefits of in-line processing.